I bought zhtham's converter box, and it arrived today. Opened it up, and inside is a receiver module, a few resistors, caps and diodes, and a 74HC04. Sound familiar? Yep, it's the bog standard circuit you find everywhere online! So if someone was thinking about DIYing one of these, you might be better off buying one of these boxes instead -- it's every bit as good as you could DIY yourself.
Oh, I suppose a DIYer could do things like use really nice parts and a high-quality power supply. Personally I plan on using a 9V power supply with a linear regultor to take it down to the 6V it wants instead of the generic wall wart that came with it. I doubt I'll mod it any further.
I still have one of those Sharp receivers coming, so I'll still build one of my own for the heck of it. I may clone this box's circuit and compare the two implementations. If I do, I'll have to trace out the schematic, so I'll probably take the time to 'capture' it and make some PDFs for y'all to scope.